Encapsulation in Java EECS2030 B: Advanced Object Oriented - - PowerPoint PPT Presentation

Encapsulation in Java

EECS2030 B: Advanced Object Oriented Programming Fall 2018 CHEN-WEI WANG

Encapsulation (1.1)

Consider the following problem:

• A person has a name, a weight, and a height.
• A person’s weight may be in kilograms or pounds.
• A person’s height may be in meters or inches.
• A person’s BMI is calculated using their height in meters and

weight in kilograms. Consider a first solution:

class Person { public String name; public double weight; /* in kilograms */ public double height; /* in meters */ public double getBMI() { return weight / (height * height); } }

• Since both attributes height and weight are declared as

public, we do not need the setter methods for them.

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Encapsulation (1.2)

Say an application of the Person class mistakenly thinks that the height in inches and weight in pounds should be set:

1 class BMICalculator { 2 public static void main(String args[]) { 3 Person jim = new Person(); 4 /* Jim’s height and weight are 1.78 m and 85 kg */ 5 jim.weight = 85 * 2.2; 6 jim.height = 1.78 * 39; 7 System.out.println(jim.getBMI()); 8 } }

• Line 7:

85×2.2 (1.78×39)2 = 0.038, rather than 85 1.782 = 26.827!!!

• Solution:

○ Disallow any application class of Person to directly assign to weight and height. ○ Provide proper setter methods as the only means for assigning these two attributes.

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Encapsulation (2.1)

Now consider a better solution:

class Person { public String name; private double weight; /* in kilograms */ private double height; /* in meters */ public void setWeightInKilograms(double k) { weight = k; } public void setWeightInPounds(double p) { weight = p / 2.2; } public void setHeightInMeters(double m) { height = m; } public void setHeightInInches(double i) { height = i / 39; } public double getBMI() { return weight / (height * height); } }

Exercise: Modify the Person class so that weight is measured in pounds and hegiht is measured in inches.

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Encapsulation (2.2)

Now an application of the Person class may only set the weight and height of a person by calling the appropriate methods:

1 class BMICalculator { 2 public static void main(String args[]) { 3 Person jim = new Person(); 4 /* Jim’s height and weight are 1.78 m and 85 kg */ 5 jim.setWeightInPounds(85 * 2.2); 6 jim.setHeightInInches(1.78 * 39); 7 System.out.println(jim.getBMI()); 8 } }

• Since both attributes weight and height in class Person are

declared as private, it is disallowed in any other class (e.g., BMICalculator) to access them (e.g., jim.weight).

• Line 7 now should return the correct BMI value.

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Encapsulation (3.1)

• Question : What if in the Person class, we want the weight

attribute to mean pounds and height to mean inches?

• Hint: Which classes will you have to change? Person?

BMICalculator? Both?

• Modify the setter methods in Person accordingly.

[Exercise!]

• No change is needed in the BMICalculator!

○ Since class BMICalculator was disallowed to access weight and height, as soon as the setter definitions are modified in Person, the calculation will still work!

• What we have achieved:

○ Implementation details in Person (i.e., weight and height) are hidden from all potential applications (e.g., BMICalculator). ○ When these implementation details are changed in Person (e.g., weight interpreted in pounds rather than in kilograms):

• Only the Person class has to be changed.
• All existing application classes can remain unchanged.

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Encapsulation (3.2)

• A software component hides the internal

details of its implementation, so that:

○ It has a stable interface; ○ Programmers of other components can

• nly depend on its public interface, rather

than writing code that depends on those implementation decisions ; ○ The component developer may change the implementation without affecting the code of any other components.

• In Java, we achieve this by

○ declaring attributes or helper methods as private; ○ providing public accessors or mutators.

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Encapsulation (3.3)

• Follow this tutorial video:

https: //www.youtube.com/watch?v=d2Q-uasRmAU&index=1& list=PL5dxAmCmjv_492h1b0yiZSyhC3ImEetLV

• For complete details about controlling the access for attributes,

refer to: https://docs.oracle.com/javase/tutorial/java/ javaOO/accesscontrol.html

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Index (1)

Encapsulation (1.1) Encapsulation (1.2) Encapsulation (2.1) Encapsulation (2.2) Encapsulation (3.1) Encapsulation (3.2) Encapsulation (3.3)

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